The invention relates to polycrystalline silicon.
Polycrystalline silicon, referred to hereinafter as polysilicon, serves, inter alia, as a starting material for the production of electronic components and solar cells.
It is obtained by thermal decomposition of a silicon-containing gas or of a silicon-containing gas mixture. This operation is referred to as deposition from the vapor phase (CVD, chemical vapor deposition).
This process is implemented on a large scale in what are called Siemens reactors. The polysilicon is obtained here in the form of rods. The polysilicon rods are generally comminuted by means of manual processes.
There are a number of known machine processes in which manually precrushed coarse chunk polysilicon is comminuted further using customary crushers. Mechanical crushing processes are described, for example, in U.S. Pat. No. 8,021,483 B2.
U.S. Pat. No. 8,074,905 discloses an apparatus comprising a device for feeding coarse chunk polysilicon into a crusher unit, the crusher unit and a sorting unit for classifying the chunk polysilicon, wherein the crusher unit is provided with a control system which enables variable adjustment of at least one crushing parameter in the crusher unit and/or of at least one sorting parameter in the sorting unit.
For applications in the semiconductor and solar industries, chunk polysilicon with minimum contamination is desirable. In order to accomplish this, various cleaning methods are also used.
US 2010/0001106 A1 describes a method for producing highly pure classified polysilicon fragments, comprising comminuting polysilicon from the Siemens method into fragments by means of a device comprising comminution tools and classifying the fragments by a screening device, and cleaning the polysilicon fragments thus obtained in a cleaning bath, wherein the comminution tools and the screening device have surfaces which contact the polysilicon and contaminate the polysilicon fragments only with extraneous particles that are subsequently removable selectively by the cleaning bath.
Silicon dust adhering to the chunks is also regarded as contamination since it reduces the yield in crystal pulling.
US 2012/0052297 A1 discloses a process for producing polycrystalline silicon, comprising crushing of polycrystalline silicon deposited on thin rods in a Siemens reactor into chunks, classifying the chunks into size classes of about 0.5 mm to greater than 45 mm and treating the chunks by means of compressed air or dry ice in order to remove silicon dust from the chunks, with no chemical wet cleaning operation.
However, the polycrystalline silicon, after the comminution steps and the optionally performed cleaning or dedusting operation, has to be packed before it is transported to the customer.
Accordingly, in the course of packing, it has to be ensured that this is effected with minimum contamination.
Typically, chunk polysilicon for the electronics industry is packed in 5 kg bags having a mass tolerance of +/−max. 50 g. For the solar industry, chunk polysilicon in bags containing a mass of 10 kg with a mass tolerance of +/−max. 100 g is standard.
Tubular bag machines suitable in principle for packing chunk silicon are commercially available. A corresponding packing machine is described, for example, in DE 36 40 520 A1.
However, chunk polysilicon is a sharp-edged, non-free-flowing bulk material having a weight of the individual silicon chunks of up to 2500 g. Therefore, in the course of packing, it has to be ensured that the material does not penetrate, or in the worst case even completely destroy, the standard plastic bags in the course of filling.
In order to prevent this, the commercial packing machines have to be modified in a suitable manner for the purpose of packing polysilicon.
U.S. Pat. No. 7,013,620 B2 discloses an apparatus for the cost-effective, fully automatic transportation, weighing, portioning, filling and packaging of high-purity polysilicon fragments, comprising a conveyor channel for the polysilicon fragments, a weighing device for the polysilicon fragments connected to a hopper, deflection plates made from silicon, a first filling device which forms a plastic bag from a highly pure plastic film, comprising a deionizer which prevents static charging and therefore contamination of the plastic film with particles, a welding device for the plastic bag filled with polysilicon fragments, a flowbox which is fitted above the conveyor channel, weighing device, filling device and welding device and which prevents contamination of the polysilicon fragments by particles, a conveyor belt with a magnetically inductive detector for the welded plastic bag filled with polysilicon chunks, all the components which come into contact with the polysilicon fragments being sheathed with silicon or clad with a highly wear-resistant plastic.
It has been found that, in such apparatuses, jamming of the silicon chunks often occurs in the filling device. This is disadvantageous, since increased shutdown times of the machine are the result.
Instances of penetration of the plastic bag also occur, and this likewise leads to a shutdown of the plant and to contamination of the silicon.
It has also been found that, during the packaging of chunks of a particular size class, for example chunk sizes of 20 to 60 mm, unwanted smaller silicon particles or chunks also arise. The proportion of such unwanted particles for such chunk sizes is 17,000-23,000 ppmw.
Hereinafter, all chunks or particles of silicon having such a size that they can be removed by means of a mesh screen with square meshes of size 8 mm×8 mm are to be referred to as fines. The fines fraction is undesirable to the customer since it adversely affects the customer's operations. If the fines fraction is removed by the customer, for example by screening, this means increased cost and inconvenience.
As well as the automatic packing of polycrystalline silicon, as according to U.S. Pat. No. 7,013,620 B2, manual packing of the polycrystalline silicon in plastic bags is also an option. Manual packing can distinctly reduce the fines fraction, for the abovementioned 20-60 mm chunk size from 17,000 ppmw down to 1400 ppmw.
However, manual packaging means high inconvenience and increased personnel costs. Therefore, manual packing is not an option for economic reasons. In addition, it would be desirable to reduce the fines fraction further than is achievable by manual packing.
It was therefore an object of the invention to automatically pack polycrystalline silicon, and to reduce the fines fraction formed to an exceptionally low level.